There is growing pressure for stationary producers of greenhouse gases to dramatically reduce their atmospheric emissions. Of particular concern is the emission of carbon dioxide (CO2) into the atmosphere. One method of reducing atmospheric CO2 emissions is through its capture and subsequent storage in geological or deep sea reservoirs.
The process for capturing CO2 from power station or combustion device flue gases is termed post combustion capture. In post combustion capture, the CO2 in flue gas is first separated from nitrogen and residual oxygen using a suitable solvent in an absorber. The CO2 is then removed from the solvent in a process called stripping (or regeneration), thus allowing the solvent to be reused. The stripped CO2 is then liquefied by compression and cooling, with appropriate drying steps to prevent hydrate formation. Post combustion capture in this form is applicable to a variety of stationary CO2 sources including power stations, steel plants, cement kilns, calciners and smelters.
Aqueous alkanolamine solutions have been investigated as solvents in post combustion CO2 capture. The capture process involves a series of chemical reactions that take place between water, the alkanolamine and carbon dioxide. Alkanolamines are weak bases, and may undergo acid-base reactions. Once dissolved into the alkanolamine solution, the aqueous CO2 reacts with water and the neutral form of the alkanolamine react to generate carbonic acid (H2CO3), aqueous bicarbonate (HCO3−) ions and aqueous carbonate (CO32−) ions, according to the generally acknowledged equations described below:CO2+2H2OHCO3−+H3O+  (equation 1)CO2+OH−HCO3−  (equation 2)CO32−+H3O+HCO3−+H2O  (equation 3)HCO3−+H3O+H2CO3+H2O  (equation 4)OH−+H3O+2H2O  (equation 5)R1R2R3N+H3O+R1R2R3NH+  (equation 6)
If the alkanolamine contains a primary (R1R2NH, R2=H) or secondary amine (R1R2NH, R2≠H), an additional reaction pathway becomes available, where carbon dioxide and the primary or secondary alkanolamine react to generate a carbamate (R1R2NCOO−). The carbamate may also then participate in acid-base chemistry, according to the generally acknowledged reactions described below. Tertiary alkanolamines (R1R2R3N, R1, R2, R3≠H) cannot form carbamates.CO2+R1R2NH+H2OR1R2NCOO−+H3O+  (equation 7)R1R2NCOO−+H3O+R1R2NCOOH  (equation 8)
It is generally acknowledged that the molar absorption capacity of an aqueous alkanolamine solution, as measured by the number of moles of CO2 absorbed per mole of amine functionality in solution, is dependent upon the pH equilibria that operate in the alkanolamine solution. CO2 absorption capacity decreases as the pH of the solution decreases. Furthermore, whilst the interrelationship of aqueous equilibria that operate as CO2 is absorbed into an alkanolamine solution is complex, it is generally accepted that the pH equilibria operating in aqueous alkanolamine solutions are to a large degree dependent upon the pKa of the alkanolamine amine functionality, and the tendency for the alkanolamine to form a carbamate. As the pKa of the alkanolamine amine increases, its aqueous solution becomes relatively more basic, resulting in a greater overall CO2 absorption capacity.
In contrast, carbamate formation by primary and secondary alkanolamines limits the molar CO2 absorption capacity of aqueous alkanolamine solutions. Low molar absorption capacities arise because carbamate formation consumes two moles of amine functionality for every mole of CO2. One mole is required to react with carbon dioxide to generate the carbamate and one mole must then act as a base to capture the proton released from the carboxylic acid functionality of the generated carbamate. This limits the molar absorption capacity nCO2/namine to a value of 0.5. Low molar absorption capacities are problematic for the application of aqueous alkanolamine solutions to industrial CO2 capture because they require more material to absorb the requisite amount of CO2, higher solvent flow rates and greater energy requirements for desorption. Monoethanolamine (MEA, HO—CH2—CH2—NH2), which is currently employed in industrial CO2 capture, possesses an undesirable molar absorption capacity of approximately 0.5.
In summary, there exists a limiting relationship between the molar absorption capacity, whether the amine functionality is primary, secondary or tertiary, and the amine pKa.
Amines used for industrial CO2 capture that achieve a larger CO2 absorption capacity than MEA have poor rates of CO2 absorption. Slow CO2 absorption rates are undesirable because to achieve the requisite absorption of CO2 longer gas-liquid contact times are required which means larger absorption columns and greater capital cost. The benefits gained through increased capacity are thus offset by the disadvantages associated with decreased rates.
There thus exists a need to identify alkanolamines whose aqueous solutions possess improved properties for application in CO2 capture technologies.